Recently, it is becoming increasingly important to reduce power consumption and cost of a semiconductor laser diode and an integrated semiconductor optical waveguide device in which a semiconductor laser diode and an electroabsorption type optical modulator are integrated. For the semiconductor laser diode, there are mainly ridge type and buried heterostructure type, of which the ridge type is advantageous for cost reduction because of its easier fabrication and smaller number of growth steps and is actively developed for use in both information and communication. The ridge type semiconductor laser diode is formed by laminating a lower clad layer, multiple or single well layer, upper clad layer, and ridge on a substrate formed of n-type semiconductor. An integrated semiconductor optical waveguide device (EA/DFB) is constructed by integrating this ridge type semiconductor laser diode and an electroabsorption type optical modulator with ridge on a common substrate.
In the field of information, high speed recording of information is demanded in accordance with an increase in the amount of information to be recorded, resulting in an increasing need for a higher power semiconductor laser diode. Although this may be met by increasing an operating current, it is disadvantageous for reduction in power consumption. On the other hand, in the field of communication, the mainstream transmission speed in current backbone network and metro network is 2.5 Gbps or 10 Gbps. Therefore, a structure of integrated semiconductor optical waveguide device in which an optical modulator is monolithically integrated for high speed modulation of a transmitter is advantageous for reduction in cost. However, power consumption increases with an increase in transmission distance as well as with the use of higher bit rate. With the aim of reducing the power consumption, the development of an electroabsorption modulator integrated distributed feedback laser (EA/DFB) that does not require temperature control between −5 degree C. and 85 degree C. has been pursued (Non-patent document 1: OFCNFOEC OFC POSTDEADLINE PAPERS Thursday, Mar. 10, 2005 PDP14). In order to achieve this, further enhancement in optical power of a semiconductor laser diode under a constant operating current is needed. Thus, the enhancement in optical power under a constant operating current is required for the reduction in power consumption of a semiconductor laser diode for use in both information and communication.
One method of power enhancement of a semiconductor laser diode is to widen its ridge width. Since the amount of heat generated becomes larger in general as the operating current in a semiconductor laser diode is raised, optical power is saturated at a certain current level, thereby making it impossible to obtain enough output. On the other hand, an electric resistance at the time of current injection into a semiconductor laser diode whose ridge width is widened is lowered, the amount of heat generation is correspondingly suppressed, and the saturation current is enhanced. As the result, the saturation output is also enhanced, and the optical power at a constant operating current is increased. The widening of the ridge width can be realized by forming an upper buffer layer between an upper clad layer and the ridge. An average refractive index difference in the lamination direction between the portion including the ridge and the portion not including the ridge becomes smaller by forming the upper buffer layer compared to when the upper buffer layer is not provided, and so-called cut-off width referred in the slab waveguide is increased, which makes it possible to widen the ridge width in lateral single mode.
As another method of the power enhancement, there is a method to suppress a rise of threshold current of a semiconductor laser diode. When the threshold current is low, optical power at a constant operating current rises, and thus the power enhancement of a semiconductor laser diode can be realized. As the method to suppress the rise of the threshold current, for example, there is a method disclosed in JP-A No. 214372/2004 (Patent document 1). In this method, a cover layer injected with Fe is formed by regrowth in both side directions of the ridge of a conventional ridge type semiconductor laser diode formed of InP series such as InP, InGaAsP and InGaAlAs, and this is used as an Fe supply source to an upper clad layer. The upper clad layer is made insulative, thereby suppressing diffusion of current injected from the ridge in the upper clad layer and a rise of the threshold current. When these lasers are made to function as a distributed feedback (DFB) type, a diffraction grating has been conventionally fabricated in an upper portion of n-substrate, a multiple well layer, or an upper buffer layer.
On the other hand, as for EA/DFB, for example, a semiconductor optical waveguide device in which a buried heterostructure type semiconductor laser diode and a ridge type electroabsorption type optical modulator are integrated differs in mode expansion in each portion, and therefore, a method of integrating the buried heterostructure type and the ridge type by tapering each joint portion is proposed in JP-A No. 78792/1996 (Patent document 2). Further, a method in which the light emitting side of a semiconductor optical waveguide device is tapered to make light coupling to fiber better is proposed in JP-A No. 66046/2000 (Patent document 3). However, no semiconductor optical waveguide device integrated with a high power laser in which ridge is widened or threshold current is lowered as described above has been disclosed.
As described above, the insertion of the upper buffer layer between the upper clad layer and the ridge is effective for power enhancement of a semiconductor laser diode, whereas there has been a problem that lateral diffusion of carrier becomes larger particularly on the p-side and the threshold current is increased. Further, since the average refractive index difference between the portion including the ridge and the portion not including the ridge becomes smaller, mode shape laterally expands, and far field pattern expansion becomes markedly different between in the horizontal direction and in the vertical direction, resulting in being asymmetrical. This causes an increase in loss of coupling to an exterior such as fiber.
To suppress the above-described rise of the threshold current, the application of the method disclosed in Patent document 1 is conceivable. However, this method requires crystal regrowth to form a cover layer. Therefore, it is disadvantageous in terms of cost reduction, and further, the application of the method is limited to InP-substrate based laser diodes, making it impossible to apply to semiconductor laser diodes formed of other materials such as GaAs-substrate based laser diodes.
For a high power semiconductor laser diode, it is effective to insert the upper buffer layer between the upper clad layer and the ridge to widen the ridge width. The application of this method to an integrated semiconductor waveguide device such as EA/DFB created another problem. For example, for power enhancement of a semiconductor laser diode, when the ridge width of a semiconductor laser diode is set to 2 μm, the capacitance increases and the band decreases in an electroabsorption type optical modulator portion. On the other hand, when the ridge width of the semiconductor laser diode is set to 1.4 μm in accord with the ridge width of 1.4 μm of the electroabsorption type optical modulator portion, the thermal characteristic of the semiconductor laser diode deteriorates and high output cannot be obtained. Therefore, as a trade-off value, the ridge width of EA/DFB has been set to ca.1.6 μm for integration which deviates from an original optimal ridge width and at which an overall characteristic deteriorates but each of the semiconductor laser diode and the electroabsorption type optical modulator can fulfill its function with ease.
Further, the insertion position of a diffraction grating also affects laser characteristics. A conventional position for fabrication of a diffraction grating has been in an upper portion of n-substrate, a multiple well layer, or an upper buffer layer. When the diffraction grating is inserted into the multiple well layer or the upper buffer layer, regrowth is carried out after forming the diffraction grating. However, a change in carrier concentration occurs at the regrowth interface, resulting in trapping of carrier, which causes deterioration of a characteristic in respect of power enhancement. On the other hand, when the diffraction grating is fabricated in the upper portion of n-substrate, the above problem can be ignored but wavelength controllability deteriorates because the diffraction grating has been formed before a multiple well layer is formed.
As described above, the above methods have not yet reached a point where reduction in power consumption and reduction in cost are compatible with each other. In addition, the design of a semiconductor laser diode and an electroabsorption type optical modulator, particularly ridge width thereof, is subject to limitation for integration, and each device has not been able to be integrated under optimal conditions.